Not that big of a fan of lip syncing a song to Anime faces - since it's fairly easy to do. The "Anime mouths" open in predictive patterns so they could fit a lot of words with their expressions, and the rest is increasing or decreasing the speed of a specific character clip to last as long as the sentence is going.

However what is amazing is the editing job that went into overlaying all of those anime characters in the video, and I can appreciate the time it took to assemble it all together.